[MM-32390] Config logic refactor (#17578)

* Replace config generator

* Cleanup

* Some renaming and docs additions to add clarity

* Cleanup logging related methods

* Cleanup emitter

* Fix TestDefaultsGenerator

* Move feature flags synchronization logic out of config package

* Remove unnecessary util functions

* Simplify load/set logic

* Refine semantics and add some test to cover them

* Remove unnecessary deep copies

* Improve logic further

* Fix license header

* Review file store tests

* Fix test

* Fix test

* Avoid additional write during initialization

* More consistent naming

* Update app/feature_flags.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

* Move FF synchronizer to its own package

* Remove unidiomatic use of sync.Once

* Add some comments

* Rename function

* More comment

Co-authored-by: Christopher Speller <crspeller@gmail.com>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

scripts/config_generator/main_test.go

@@ -0,0 +1,41 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package main
+
+import (
+	"encoding/json"
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"github.com/mattermost/mattermost-server/v5/model"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDefaultsGenerator(t *testing.T) {
+	tmpFile, err := ioutil.TempFile("", "tempconfig")
+	defer os.Remove(tmpFile.Name())
+	require.NoError(t, err)
+	require.NoError(t, generateDefaultConfig(tmpFile))
+	_ = tmpFile.Close()
+	var config model.Config
+
+	b, err := ioutil.ReadFile(tmpFile.Name())
+	require.NoError(t, err)
+	require.NoError(t, json.Unmarshal(b, &config))
+	require.True(t, *config.ServiceSettings.DisableLegacyMFA)
+	require.Equal(t, *config.SqlSettings.AtRestEncryptKey, "")
+	require.Equal(t, *config.FileSettings.PublicLinkSalt, "")
+
+	require.Equal(t, *config.Office365Settings.Scope, model.OFFICE365_SETTINGS_DEFAULT_SCOPE)
+	require.Equal(t, *config.Office365Settings.AuthEndpoint, model.OFFICE365_SETTINGS_DEFAULT_AUTH_ENDPOINT)
+	require.Equal(t, *config.Office365Settings.UserApiEndpoint, model.OFFICE365_SETTINGS_DEFAULT_USER_API_ENDPOINT)
+	require.Equal(t, *config.Office365Settings.TokenEndpoint, model.OFFICE365_SETTINGS_DEFAULT_TOKEN_ENDPOINT)
+
+	require.Equal(t, *config.GoogleSettings.Scope, model.GOOGLE_SETTINGS_DEFAULT_SCOPE)
+	require.Equal(t, *config.GoogleSettings.AuthEndpoint, model.GOOGLE_SETTINGS_DEFAULT_AUTH_ENDPOINT)
+	require.Equal(t, *config.GoogleSettings.UserApiEndpoint, model.GOOGLE_SETTINGS_DEFAULT_USER_API_ENDPOINT)
+	require.Equal(t, *config.GoogleSettings.TokenEndpoint, model.GOOGLE_SETTINGS_DEFAULT_TOKEN_ENDPOINT)
+}